1. Field of the Invention
My invention relates to the field of encrypted electronic communications, and more specifically to encrypted spread spectrum conference communications in a jamming environment without yielding usable depths.
2. Description of the Prior Art
A common use of spread spectrum systems in the prior art has been as a multiple access or selective address technique for communications among numbers of ground stations via a single satellite channel. A direct sequence spread spectrum system allows many signals to be transmitted on the same nominal carrier frequency and occupy the same radio frequency bandwidth. FIG. 1 illustrates such a system which might be used in a satellite link. A first transmitter, represented by a modulator 103 and mixer 104, sends a signal via satellite 105 to a receiver, represented by mixer 108 and demodulator 109. A second transmitter and receiver are also shown, and multiple paths are usual. A carrier signal s.sub.o (t)=A cos .omega..sub.o t is modulated with information in modulator 103 to produce a signal s.sub.1 (t), where EQU s.sub.1 (t)=A.sub.1 (t) cos [.omega..sub.o t+.phi..sub.1 (t)].
The modulated signal s.sub.1 (t) is next linearly multiplied in mixer 104 by some other function of time g.sub.1 (t). The function g.sub.1 (t) is usually the function that spreads the energy of the modulated signal s.sub.1 (t) over a bandwidth considerably greater than the bandwidth of s.sub.1 (t).
The resulting twice-modulated signal, given by the product g.sub.1 (t).multidot.s.sub.1 (t), is transmitted over a common channel such as a satellite link where it is linearly combined with the other signals in the system, i.e., g.sub.2 (t).multidot.s.sub.2 (t), g.sub.3 (t).multidot.s.sub.3 (t) , . . . , g.sub.n (t).multidot.s.sub.n (t). All these signals have the same nominal carrier frequency.
The composite signal, formed by the sum of all signals g.sub.i (t) s.sub.i (t) transmitted, is applied to the input of that receiver whose function is to extract the original signal s.sub.1 (t) and reject all others. To do this the composite signal is multiplied in mixer 108 by an exact replica of g.sub.1 (t). This process produces the following signal at the output of mixer 108: EQU g.sub.1.sup.2 (t).multidot.s.sub.1 (t)+g.sub.1 (t).multidot.g.sub.2 (t).multidot.s.sub.2 (t)+g.sub.1 (t).multidot.g.sub.3 (t).multidot.s.sub.3 (t)+ . . . +g.sub.1 (t).multidot.g.sub.n (t).multidot.s.sub.n (t).
If the functions g.sub.i (t) can be chosen such that g.sub.1.sup.2 (t)=1 and cross products such as g.sub.i (t).multidot.g.sub.j (t)=0 where i.noteq.j, then each demodulator would be able to perfectly extract its own signal and reject all others. The process just described is correlation, and useful approximations to functions having the necessary relationships may be produced by shift register sequence generators.
A well known method of binary data encryption involves the modulo-2 addition of a binary data stream to a key stream comprising a sequence of pseudorandom binary bits to produce a cipher stream. The cipher stream is transmitted to a receiver where it is modulo-2 added to an identical key stream with the result that the original binary data stream is reproduced. It is important to the security of such a system that no two data streams be encrypted with an identical key stream, because the addition of the two resulting cipher streams eliminates the key, leaving only the sum of the two data streams. Because of the highly biased nature of most data streams it is usually a simple process to separate the two and discover the information which was intended to be kept hidden. Cryptanalysts commonly refer to such a technique as depth exploitation. A depth refers to any repetition of a key stream, or a portion of a key stream, which permits an unauthorized recipient of the transmission to gain an insight into the manner in which the key was generated. Given a sufficient number of depths it is possible to reconstruct the method by which the key stream was generated and thereafter decipher all of the subsequent communications.
Depth exploitation has particular significance for conferencing systems in which it is intended that several persons be able to communicate over a common channel and at the same time. A typical military application might include a squadron of aircraft on a common mission. A business conference call poses a non-military situation where it might be desirable for several persons to converse on a common channel, and it is increasingly desirable that such calls be encrypted. The simple expedient of encrypting all messages in a conferencing network by means of a single common key results in the creation of depths any time two or more persons are talking at the same time. There is a need for a communications system which provides encrypted conferencing capabilities that do not exhibit a high potential for depth exploitation.